While the usual polyester used as a photographic film base is generally polyethyleneterephthalate, other synthetic linear polymers in the polyester class have also been used at one time or another for the same purpose and may find their way into discarded forms which may be designated as scrap.
Synthetic linear polyesters are well known in the art and the present invention is applicable to the general class of synthetic linear thermoplastic polyesters. A polyester is defined as a synthetic linear condensation type polymer whose repeating units contain the ester group, ##STR1## these groups being integral members of the linear polyester chain. Polyesters may be those derived from aliphatic dibasic acids such as oxalic, succinic, glutaric, adipic, and sebacic acids and glycols such as ethylene glycol, propylene glycol, trimethylene glycol, hexamethylene glycol and decamethylene glycol. Polyesters may also be derived from aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid and glycols such as ethylene glycol. Polyesters may also be derived from hydroxy acids and their corresponding lactones such as those from hydroxypivalic acid, .alpha.-hydroxyisobutyric acid, .omega.-hydroxycaproic acid, .omega.-hydroxydecanoic acid, .gamma.-butyrolactone and 4-hydroxyhexanoic acid lactone.
An important source of silver-bearing polyester material is scrap photographic film. When the photographic film is based on silver halide, this scrap contains a polyester base, silver halide and/or metallic silver, gelatin, and a subbing layer on the polyester surface consisting principally of polyvinylidenechloride and usually comprising a terpolymer containing a major amount of vinylidenechloride, a lesser amount of acrylonitrile, and a relatively small amount of a polymerizable acid taken from the class of itaconic, acrylic or methacrylic acids of the alkyl esters thereof. The scrap may also contain casual dirt and various dyes or pigments and may also be contaminated with cellulose acetate film.
Substantially all of the prior art dealing with solvent treatment of scrap materials such as photographic film having a linear thermoplastic polyester as a film base, have as their principal object of the recovery of the polyester values only in useful form. The prior art which deals with recovery of silver values from scrap photographic film invariably utilizes techniques which remove the silver values preferentially, leaving the polyester containing values in effectively unchanged condition, or in other words, in the same shape and form as originally supplied to the recovery process. Thus, for example, the techniques are normally deliberately chosen so that the polyester values and in many cases the polyvinylidenechloride values remain insoluble in the solution treatment. This preferential removal of silver is also utilized in the prior art even when the polyester is placed in solution for recovery purposes.
Known procedures as described in U.S. Pat. Nos. 3,647,422 issued Mar. 7, 1972; 3,873,314 issued Mar. 25, 1975 and 3,928,253 issued Dec. 23, 1975 for example, include destruction of the gelatin layer for removal of the silver value in a filterable condition so that it can be recovered as a precipitate by (1) treatment of the film with a proteolytic enzyme which dissolves the gelatin thus freeing the silver; or (2) swelling and dissolving the gelatin layer and releasing the silver with the use of hot caustic solutions; or (3) treatment of the scrap film with hot ethylene glycol, ethanolamine and the like.
In some cases, the polyester chips thus recovered are sometimes obtained in a useful condition for reuse but usually sufficient contamination still remains so that the degree of reuse is severely limited. For example, the polyester film base may contain pigments and dyes. Further, some of the treatments leave a significant portion of the polyvinylidenechloride terpolymer remaining on the surface of the polyester which is a strongly degrading influence when the recovered polyester is to be utilized as a thermoplastic raw material for the preparation of full value polyester articles.
In other cases, the scrap photographic film may contain significant amounts of cellulose triacetate which the treatments described heretofore in the prior art for silver recovery do not remove. Again, the presence of this cellulose triacetate is an extremely deleterious contaminant which prevents adequate utilization of the recovered polyester.
In brief, the present novel process for recovery of values from scrap photographic film is based on treatment of the silver containing film with specific solvents under controlled conditions in which the solvents act as solvents for all the non-water soluble polymers and in which the gelatin and silver fractions are insoluble, and from which the non-water soluble polymers can be separated and recovered as individual polymers.
In order for the described solution process to be fully effective for the purposes of this invention not only must all silver values be recovered quantitatively but also the polymeric materials other than gelatin require recovery in fully useful form through the medium of economic and quantitative procedures. The most useful form in which the non-water soluble polymer can be recovered is as a powder of controlled particle size with particular reference to the polyester.
Equally important is the requirement for solvent stability. Economic and chemical considerations require that there be little or no decomposition of the solvent media or significant reaction of the solvent media with the photographic film raw material, since it is an economic necessity that the solvent be capable of being recovered in fully useful form for recycling through the process effectively indefinitely. Decomposition or undesired modification of the solvent, even if of small extent, reduces this required recycle capability, increases the cost of solvent recovery, reduces the amount of solvent which can be recovered for useful purposes, and is some cases decomposition is sufficiently extensive so as to limit severely or even prevent the reuse of the solvent for recycle purposes. Thus, many solvents which are adequate from the standpoint of placing the non-water soluble polymers in solution initially to permit the desired partition of the various components of the photographic film, are undesirable for the purposes of this invention because of the extensive decomposition which such solvents undergo in the preferred process conditions.
Further in addition to solvent stability, the mutual solvents used for dissolving the non-water soluble components of the photographic film scrap should exhibit a solubility for the polyester portions of the film scrap preferably not exceeding 3 grams of polyester per 100 cc's of solvent, when the temperature of the system is in the range of 25.degree. to 35.degree. C and again preferably should exhibit substantially zero but not more than 0.5% solubility for the polyester portions of the film scrap when the mixture of mutual solvent, residual polyester and other non-water soluble polymers is cooled to a temperature range between 0.degree. to -10.degree. C. It is a further requirement that the mutual solvent utilized exhibit a solubility of at least 10 grams per 100 cc's of mutual solvent for the polyvinylidenechloride portion, and other non-water soluble polymer over a temperature range between -10.degree. C and +35.degree. C, and that the solubility of these components be considerably higher at temperatures above 35.degree. C.
Further, in order for a solvent procedure for the preparation of polyester powder by precipitation from solution to be effective economically and quantitatively, there should be no degradation in molecular weight of the raw material fed to the dissolving medium or, in other words, the intrinsic or relative viscosity of the final product obtained in powdered form should not be less than the similar parameter of the starting raw material. In addition, the recovery of this non-degraded polyester needs to be reasonably quantitative.
Thus, in order to meet all of the requirements heretofore indicated with regard to stability of solvent, solubility of polyester and polyvinylidenechloride and other non-water soluble polymers, maintenance of molecular weight of the recovered polyester in pure form and other requirements, the varieties and members of mutual solvents which meet all of these requirements without exception become highly restricted particularly when one recognizes that a common contaminant of scrap silver bearing photographic film may be cellulose acetate.
Thus, solvents which have been designated in the prior art as being primarily suitable for placing polyester in solution even under preferred conditions suffer substantially from one or more defects which place them outside of the preferred requirements. In some cases, the polyester is precipitated by quenching in various liquids, many of which are in themselves solvents for polyester at room temperature above the 3% limit recited above. Examples of such solvents are those involving mixtures of halogenated aliphatic acids either alone or combined with chlorinated hydrocarbons, phenols such as chlorinated phenol, phenol itself and metacresol, again either alone or in combination with chlorinated hydrocarbons. In addition if these types of solvents are utilized under the conditions described in this invention profound degradation of the molecular weight of the polyester component takes place. Hydrocarbons such as diphenylmethane, biphenyl, acenaphthene, phenanthrene and naphthalene which are designated in the prior art as solvents for polyester are also unsuitable since the cellulose acetate contamination is insoluble in such solvents under the preferred conditions defined by this invention.
Many other solvents are defective not only for the foregoing reasons but also because they are powerful degraders of the molecular weight of polyester when utilized under the preferred conditions typified by the process of this invention. Such solvents which in addition to other defects cause molecular weight degradation include the halogenated aliphatic acids, the phenols, the cresols, benzyl alcohol, ethylene glycol, hexafluoroisopropanol, and others.
Certain other solvents listed in the prior art as appropriate solvents for polyester undergo significant decomposition when utilized in accordance with the process of this invention, including dimethylsulfoxide, quinolines, organic sulfides, substituted amines and other heterocyclic nitrogen compounds. However, amides are satisfactory. Further, the adverse decomposition appears to be typical of halogenated solvents generally. The evidence indicates that the degradation reaction is initiated by the silver or gelatin component or both. Even when the degradation of solvent is slight for whatever reason and outside of the difficulties imposed on adequate solvent recovery, a principal result is the recovery of a disclosed polyester end product which should be glistening white with no off-white rendition in order to ensure full utility.
Finally, since more than one non-water soluble polymer is placed into complete solution under the preferred conditions of the process of this invention, it is a requirement that means for separating the individual components be available so that each of these non-water soluble polymeric materials may be separated from the other and recovered in pure powdered form.
As a consequence of this wide group of restrictions on the nature, performance and character of the mutual solvent for all of the non-water soluble polymeric constituents, materials which meet these requirements become severely limited and the attaining of the desired results is not only a question of choice of the appropriate solvent or mixture of solvents but the manner in which these solvents are processed.
The availability of undegraded polyester powder of controlled particle size represents a distinct advantage over recovered polyester which retains its original shape even though the recovered polyester in its original shape may be designated as pure. While both powdered polyester and polyester in chip form may be utilized for forming shaped articles from the melt, the powder form possesses a number of advantages over chips. Powder is more readily compounded prior to the melting operation. The molecular weight of polyesters can be upgraded by heat treatment in a vacuum and/or in an inert atmosphere at temperatures below the melting point for extended periods of time, and the finer the particle size of the polyester utilized in this molecular upgrading process, the faster this molecular weight upgrading can be achieved; and, finally, the powdered form is a necessity for the production of a compounded surface finish. The production of powder of the desired particle size from bulky forms by mechanical attrition procedures is extremely difficult, costly and sometimes impossible in view of the toughness of the starting raw material.